Multiple people will use your app at the same time, and you want to deliver your app as fast as possible. So you'll need some way to handle concurrency. Fear not! Most web servers already do this by default. But when you need to scale, you want to use concurrency in the most efficient way possible.

Working on one commercial project I had to import and parse a xls file. Since the file had huge amount of data the parsing took time. To overcome this time-consuming process I had to split a parsing thread on a few ones.